Heart valve prosthesis

ABSTRACT

The prosthesis comprises a unitary annular valve body having a pair of leaflets (2, 3) mounted on pivot posts adjacent their outer edges, so as to pivot between a closed position (in which the free edges of the leaflets (2, 3) are contiguous so as to inhibit flow of blood through the valve body) and a fully open position (in which the free edges are parallel to one another so as to permit flow of blood through the valve body). The valve body and the leaflets (2, 3) are each precision moulded, homogenous monolithic vitreous carbon artifacts having an as moulded surface of optical quality.

The present invention is concerned with heart valve prostheses.

Various types of synthetic heart prosthesis are known which operatehaemodynamically when used in the surgical replacement of a heart valve,such as the mitral or aortic valve.

One known type of heart valve prosthesis is the St. Jude valve, whichcomprises a valve body having a passageway for the flow of bloodtherethrough from upstream to downstream, and a pair of flat leafletswhich are pivotally supported in the passageway and movable between aclosed position inhibiting blood flow and an open position permittingblood flow therethrough.

Another known type of heart valve prosthesis is the Duramedics valvewhich is a bileaflet valve in which the leaflets are curved in a planeparallel to the pivot axis of the leaflets.

Such heart valve prostheses have peripheral pivot points, the pivot axesbeing substantially parallel to the junction between the two leaflets,and each axis defining a chord on the respective leaflet. Such heartvalve prostheses all have points of weakness at the pivot points, whichcan cause catastrophic failure and/or promote blood coagulation.Moreover, the leaflets provide obstructions to the flow of blood throughthe valve and substantially disturb the laminarity of such blood flow.

It has been previously proposed, in broad terms, that vitreous carbonwould be a suitable material for the production of heart valveprosthesis. Vitreous carbon artefacts are produced by careful,controlled pyrolysis of resin mouldings is described in "PolymericCarbons, Carbon Fibre, Glass and Char" by G. Jenkins and K. Kawamura,Cambridge University Press, 1976. In this method, the resin material(such as a phenolic resin or furan resin) is cast and cured as a block,machined to the form of the final artefact, and then slowly pyrolysedover a prolonged period.

We have now devised an improved bileaflet heart valve prostheses, whichenables the properties of vitreous carbon, both during the pyrolysisprocess, and in the resulting artefact, to be optimised.

According to the present invention, therefore, there is provided a heartvalve prosthesis comprising a unitary annular valve body having apassageway for the flow of blood therethrough from upstream todownstream, and a pair of leaflets pivotally mounted in said valve bodyfor pivotal movement between a closed position, in which closed positionsaid leaflets lie contiguous with one another so as to obturate saidpassageway to the flow of blood therethrough, and a fully open position,in which the inner edges of said leaflets are substantially parallel toone another in a downstream position, so as to permit blood flow throughsaid passageway, each of said leaflets being pivotally mounted adjacentthe outer edge thereof about a pivot axis which is substantiallyparallel to said inner edges and which does not intersect the respectiveleaflet, in which each of said leaflets and said valve body is aprecision moulded, homogeneous monolithic vitreous carbon artefacthaving an as-moulded surface of optical quality.

The valve body and the leaflets have, in the as-moulded state, all thesurface characteristics of the mould surfaces; further machining orpolishing is generally unnecessary and, indeed, undesirable. The mouldsurfaces employed to produce the vitreous carbon artefacts are thereforeof optical quality. By optical quality, we mean herein a surface finishwith a centre-line average (Ra) of less than 0.05 micrometers.

The annular valve body and the leaflets are all homogeneous (that is,they are essentially free of granular or powdered filler, fibrousreinforcement, or other inclusions). They are further monolithic, beingdevoid of any coating, and have a purity of substantially 100% (that is,less than about 50 ppm of impurities). The valve body and the leafletsare generally impermeable to helium and heavier gases.

The bileaflet heart valve prosthesis according to the invention isgenerally such that the passageway formed when the leaflets are in thefully open position is substantially free of any transverse obstructionto the blood flow; it is particularly preferred that the spacing betweenthe inner edges of the leaflets is at least as great as the spacingbetween the axes about which the leaflets pivot.

The leaflets are preferably symmetrical and generally semi-circular orsemi-elliptical when viewed in plan; it is particularly preferred thateach of the leaflets is shaped for substantially non-turbulent passageof blood along both faces thereof when the leaflets are in the fullyopen position. It is preferred that both faces of the leaflets should bein the form of continuous smooth curves; it is further preferred that inthe closed position the leaflets should present a generally concavesurface when viewed from upstream.

In the latter closed position, the leaflets are preferably whollycontained within the upstream and downstream limits of the valve body,as are the relevant pivot structures. The latter are preferably upstreamof the median plane of the valve body, whereby the protrusion of theleaflets from the valve body in the fully open position can beminimised, with consequent minimisation of impongement of the leafletswith muscle and other tissue.

The valve body will generally be held in place by means of a peripheralsewing ring; the latter is preferably retained on the valve body withthe aid of a plurality of spaced grooves provided around thecircumferential periphery of the valve body.

In one preferred embodiment of the invention, the pivot axes of therespective leaflets are substantially parallel to opposed external facesof the valve body. It is further particularly preferred that the pivotaxes should be substantially perpendicular to opposed transverseexternal faces of the valve body (that is, there are preferably twopairs of generally parallel external faces for the valve body); adjacentones of the external faces are generally connected by smoothly curvedportions, such that the whole external shape is generally rectangularwith curved corners. It is a feature of the present invention that theexternal faces of the valve body which are parallel to the pivot axes ofthe leaflets may be longer than the external faces perpendicularthereto; that is, unlike the conventional circular valve body, the valvebody employed in the valve according to the invention need not beaxially symmetrical.

Such a geometry enables the "effective orifice area" (that is, the ratioof the internal diameter to the external diameter) to be increasedrelative to a conventional circular occluder ring. This enables the sizeof valve which can be implanted in a patient to be maximised, therebyoptimising the volume of blood flowing through the valve during use.

The vitreous carbon artefacts (that is, the valve body and the pair ofleaflets) are generally produced by a process comprising partiallycuring a substantially water-free phenolic resin in ambient atmosphere;precision moulding the partially cured phenolic resin in an enclosedmould having moulding surfaces of optical quality (as defined above),which are preferably such that the resulting moulding has a maximumthickness not exceeding 6 millimeters, under such conditions that theresulting moulding is substantially fully cured, substantiallypore-free, and transparent; gradually heating the resulting moulding ina non-oxidising atmosphere over a period of at least twenty hours tofinal temperature of at least 1000° C.; and maintaining the finaltemperature until the moulding is substantially fully carbonized tovitreous carbon.

In the phenolic resin used in the method just described, thephenol/aldehyde ratio is generally slightly greater than 1:1 (such as1.1 to 1.8:1), that is, a resol. An inappropriate phenol:aldehyde ratiomay cause cracking of the final artefact. The phenol may be phenolitself, a cresol, xylenol or the like. The aldehyde is typicallyformaldehyde. Before the step of partial curing in ambient atmosphere,the resin should be rendered as free of water as possible so as tominimise porosity in the resulting artefact (if water is not eliminated,the resulting artefact could contain pores typically of size around 50microns).

The water may be removed by any suitable method, preferably by thecombined action of heat and reduced pressure (for example, in a rotaryevaporator) whereby the water will be boiled off; alternatively, anazeotropic mixture can be formed which may allow the water to be removedat a lower temperature.

The moulding stage (typically compression or transfer moulding) mayemploy moulds with glass, polished metal, or other suitable opticalquality surfaces. Metal surfaces, such as surfaces of high qualitysteel, are preferred.

Transfer moulding is preferred, and is preferably carried out so thatthe resin is raised to a temperature of greater than 100° C., forexample, about 160° C., such that substantially complete cure can takeplace within the mould. Shortly after removal from the mould, themoulding may, if desired, be given a permanent deformation.

The carbonisation of the moulding can be considered as three successivestages. At lower temperatures (such as 300° C. to 500° C.), there isconsiderable gas evolution, and consequent weight loss; at highertemperatures (such as from 450° to 600° C.), there is rearrangement ofthe molecular structure and consequent shrinkage; while at even highertemperatures residual hydrogen is driven off.

The rate of temperature rise during carbonisation should be closelycontrolled, the optimum rate depending on the thickness and otherdimensions of the artefact, and the temperature. Artefacts with athickness greater than 6 millimeters cannot be satisfactorilycarbonised, whereas for artefacts with a thickness of about 4millimeters, the temperature rise at lower temperatures (such as up to650° C.) is typically of the order of a few degrees Celsius (such as5°-20° C.) per hour. The rate of temperature increase at highertemperatures is less critical, and at such higher temperatures the rateof increase may be of the order of a hundred degrees Celsius per hour.

The non-oxidising atmosphere may be an inert gas such as argon, heliumor nitrogen, optionally in admixture with a minor amount of hydrogen.

The fact that the method described above involves substantial shrinkageduring carbonisation is advantageous, as will now be described.Specifically, the pair of leaflets are initially formed with integraltrunnion structures adjacent an edge of each leaflet permittingpivotting of the leaflets in an appropriately shaped annular valve body;the valve body is then formed and shrunk around the leaflets such thatcomplementary formations in the occluder ring seat the trunnionstructures provided on the leaflets to permit pivotting of the leaflets.The pivot axis about which a respective trunnion structure is pivotal issubstantially parallel to the inner edge of the leaflet and does notintersect the respective leaflet. That is, each pivot axis is eithertangential to, or outside the periphery of, the respective leaflet.

This method of production involving controlled shrinkage offers a highdegree of structural integrity for the resulting valve structure, andminimises the possibility of inadvertent separation of a leaflet fromthe valve body; no further securing of the valve leaflets is thereforerequired.

Preferred embodiments of the present invention will now be describedwith reference to the accompanying drawings, in which:

FIG. 1 is a plan view of an exemplary heart valve according to theinvention with (for illustration purposes only) one leaflet in the openposition and the other in the closed position;

FIG. 2 is a sectional view through the valve of FIG. 2; and

FIG. 3 is a sectional view through an alternative valve in which theleaflets are in the fully open position.

Referring to FIGS. 1 and 2, the illustrated valve comprises a valveannulus 1 (also known as an occluder ring) having a pair of generallysemi-circular (when viewed in plan) leaflets 2,3 each having aperipheral extension 4,5 at the edge thereof. The peripheral extensionsserve as pivot posts or trunnions, and are seated in respective seatingstructures 6,7 provided in valve annulus 1.

Leaflets 2,3 are concave when viewed in the direction of arrow A (fromthe upstream direction when in their closed position) and convex whenviewed from downstream. Each face is in the form of a smooth continuouscurve, such that blood can flow in a non-turbulent manner over bothfaces when the leaflets are in their open positio.

In the embodiment of FIGS. 1 and 2, the valve annulus 1 is generallyrectangular in plan view, having a first pair of parallel external faces10,11 which are parallel to the axes of extensions 4,5 and a second pairof external faces 12,13 perpendicular thereto. The corners 14,15,16,17are smoothly rounded off.

Referring to FIG. 3, the alternative heart valve illustrated hasdifferent seating structures 6,7 to those of FIGS. 1 and 2. The valve isshown with both leaflets 2,3 in the full open position. In this positionit will be noted that the free edges 8,9 are separated by an amountequal to the spacing between the pivot posts or trunnions. In thisposition, the passageway through the valve is, as shown, free of anytransverse obstruction to the flow of blood therethrough.

We claim:
 1. A heart valve prosthesis comprising a unitary annular valvebody having a passageway for the flow of blood therethrough fromupstream to downstream, and a pair of leaflets pivotally mounted in saidvalve body for pivotal movement between a closed position, in whichclosed position said leaflets lie contiguous with one another so as toobturate said passageway to the flow of blood therethrough, and a fullyopen position, in which inner edges of said leaflets are substantiallyparallel to one another in a downstream position, so as to permit bloodflow through said passageway, each of said leaflets being pivotallymounted adjacent an outer edge thereof about a pivot axis which issubstantially parallel to said inner edges and which does not intersectthe respective leaflet, in which said leaflets and said valve body areprecision moulded, homogeneous monolithic vitreous carbon artifactshaving an as-moulded surface of optical quality.
 2. A heart valveprosthesis according to claim 1, wherein the passageway formed when theleaflets are in the fully open position is substantially free of anytransverse obstruction to the blood flow.
 3. A heart valve prosthesisaccording to claim 1, wherein the spacing between said inner edges whensaid leaflets are in said fully open position is not substantially lessthan the spacing between the axes about which the leaflets pivot.
 4. Aheart valve prosthesis according to claim 1 wherein said leaflets aresymmetrical and generally semi-circular or semi-elliptical when viewedin plan.
 5. A heart valve prosthesis according to claim 1, wherein facesof each leaflet are in the form of respective continuous curves.
 6. Aheart valve prosthesis according to claim 1, wherein said leafletspresent a generally concave surface in the closed position when viewedfrom upstream.
 7. A heart valve prosthesis according to claim 6, whereinleaflets are wholly contained within upstream and downstream limits ofthe valve body when in the closed position.
 8. A heart valve prosthesisaccording to claim 1, in which said pivot axes are substantiallyparallel to opposed external faces of said valve body.
 9. A heart valveprosthesis according to claim 1, in which said pivot axes aresubstantially perpendicular to opposed transverse external faces of saidvalve body.
 10. A heart valve prosthesis according to claim 1, in whicha plurality of spaced grooves are provided around the circumferentialperiphery of the valve body.